The invention relates to an improved system and method for voltage-controlled oscillator (VCO) calibration in a phase-locked loop (PLL) for time base generator applications. In particular, the invention calibrates a VCO center frequency to a reference frequency in a dynamic integrated circuit environment.
VCO circuits are well known in the art and are typically used in applications where a stable, controllable, high frequency signal is required. For example, a VCO may be used as part of a PLL in a frequency synthesizer to provide high frequency clock signals. PLLs use phase to lock on to a signal to generate synchronous clocking of digital signals. In a typical application, these clock signals may be applied to the clock inputs of integrated circuit chips in a particular system to synchronize their operations.
It is well known in the art of integrated circuit design to use fuses (low resistance, e.g. 200.OMEGA., connections in an unblown state) in conjunction with other circuit elements to adjust circuit operating parameters, such as voltage or current gain. See e.g., U.S. Pat. No. 5,412,594. By selectively blowing fuses, circuits may be "trimmed" after manufacture to accommodate desired parameters. When blown, fuses become high resistance elements (e.g., 10 K.OMEGA. to 20 M.OMEGA.), thus isolating other elements or sections from the rest of the circuit. In particular, fuse trimming circuits have traditionally been used to reduce the overall center frequency error in VCO circuits due to post-manufacture non-ideal circuit and process variations.
Such prior art fuse trimming circuits inherently have several disadvantages, however. First, the fuse trimming method increases production costs. Because it is necessary to physically contact the fuse with a probe in order to blow it, fuse trimming typically occurs as part of wafer probe during the manufacturing process. In the case of VCO adjustment, frequency measurements will also have to be performed during wafer probe as part of the fuse trimming method. Also, additional test time will be required to achieve accuracy in high frequency measurements.
Second, because fuse trimming is an irreversible, one-time process, the trimmed circuit is limited to a particular data rate and nominal conditions. Data rate changes occur, for example, when a computer system changes zones on a disk drive. As the head of the disk drive moves from the outer zone of the disk to the inner zone, the data rate slows down. Data rate changes and other dynamic characteristics, must therefore fall within limited tolerances allowed by the specifications of the trimmed circuit, because it will be difficult or impossible to further adjust the circuit after wafer probe has been completed.
Moreover, many applications pack these integrated circuits in plastic or other encapsulation materials. Such packaging may vary circuit parameters after trimming has occurred. While reverse trimming methods have been developed in an effort to counter these problems, such methods do not always resolve satisfactorily many inherent circuit inconsistencies.
Third, fuses that are blown or marginally-blown exhibit varying resistance characteristics from chip to chip. In addition, marginally-blown fuses may exhibit lower resistivity as a result of regrowth. VCO circuits are sensitive to "open" fuse resistance, which typically varies from 10 K.OMEGA. to 20 M.OMEGA.. Such broad variation in resistivity limits a designer's flexibility when designing compatible circuits.
Finally, given the uncertainty of blown or marginally-blown fuses, circuit reliability problems exist.